A fuel cell is a direct energy conversion device that generates electricity directly from a fuel source, such as hydrogen gas, and an oxidant, such as oxygen or air. Since the energy conversion process does not “burn” the fuel to produce heat, the thermodynamic limits on efficiency are much higher than normal power generation processes. In essence, the fuel cell consists of two catalytic electrodes and an electrolyte which can be an ion-conducting membrane. The fuel gas (e.g. hydrogen) is oxidized on the anode forming hydrogen ions, and the hydrogen ions diffuse across the membrane to recombine with the oxygen ions on the surface of the cathode. A potential gradient is generated, driving electrons through an external electrical circuit or a load, producing electrical power.
Conventional fuel cell stacks consist of a number of individual components (metal or graphite bi-polar plates, membrane electrode assemblies, gaskets, etc) that are held together by compressive forces. Typically, four separate seals are needed for each cell, and to make those seals tight, significant compressive force on the gaskets is needed. To obtain the necessary compressive force, end plates and screws or bolts are required, adding considerable weight to the fuel cell stack. To achieve good contact between all of the fuel cell pieces, you need significant force that must also be uniform over the entire electrode area, which leads to thick, heavy endplates and lots of strong tie-rods connecting the endplates, squeezing the stack.
Conventional stacks work best when they have large areas and are assembled in relatively short stacks—not too many cells in series. This configuration gives high currents, and low stack voltages. If one cell fails, the stack fails.